1. Field of the Invention
The present invention relates to optical wafer inspection systems. More particularly, the invention relates to apertures and shaping of the imaging path in optical wafer inspection systems.
2. Description of Related Art
Optical apertures with flexible sizes and/or shapes are useful in manipulating illumination and imaging properties in optical wafer inspection systems. Optical apertures with such flexibility allow illumination and imaging properties to be optimized given a selected wafer pattern and/or selected types of defects of interest. Varying sizes and shapes of optical apertures can be applied in both brightfield and darkfield optical modes. The use of special aperture sizes and/or shapes are also known to enhance signal to noise ratios in optical wafer inspection systems.
Thus, there is a need for flexible and programmable methods for selecting optical aperture sizes and/or shapes. In addition, the optical apertures may maximize optical transmission in the open portion (as close as possible to 100% transmission) for light transmission efficiency while minimizing optical transmission in the blocked portion (as close as possible to 0% transmission) to prevent light leakage. It may also be desired for the optical apertures to minimize stray light, minimize optical aberrations, withstand high optical power densities (e.g., from near infrared (NIR) to visible (VIS) to deep ultraviolet (DUV) to vacuum ultraviolet (VUV)), and inhibit contamination (especially in DUV and VUV optics). While the flexible optical apertures may be capable of providing fully open/fully blocked shapes, the apertures may also allow the inclusion of shaped optical elements with variably attenuating, polarizing, spectral, phase, and/or gradient properties (e.g., polarizing apertures, apodized apertures, dichroic apertures, or phase plates).
One method used for providing flexible optical apertures is an iris mechanism with adjustable blades, similar to apertures used in cameras or other optical instruments. FIG. 1 depicts an example of an embodiment of an iris mechanism. A base plate, blades, and a blade actuating ring are shown in the top row. The bottom row depicts three stages of blade adjustment—open position, blades in motion, and half-open position. In the iris mechanism, aperture size can be varied by moving the blades between the open position and a closed position. The shape of the aperture, however, is limited (e.g., typically the blades create an aperture approximating a circular aperture) and there is no allowance for the inclusion of optical elements in the iris mechanism.
Another method used for providing flexible optical apertures is an aperture wheel (e.g., rotating aperture wheel). FIG. 2 depicts an example of an embodiment of an aperture wheel. The wheel shown in FIG. 2 includes multiple apertures that can be rotated into position to define the illumination. The number of types of shapes that can be accessed on the wheel, however, is limited by the apertures on the wheel.
FIG. 3 depicts an example of an embodiment of a linear slider with multiple apertures used for providing flexible optical apertures. The linear slider can be moved (e.g., slid) to place an aperture in position to define the illumination. FIG. 4 depicts an example of an embodiment of a tape drive with multiple apertures used for providing flexible optical apertures. The tape drive includes a thin tape with apertures that is rotated using reels on both ends of the tape to position a selected aperture to define the illumination. Similar to the aperture wheel, the number of types of shapes that can be accessed on the linear slider or the tape drive, however, is limited by the apertures on the slider or the tape drive.
Liquid crystal arrays (e.g., matrices) have been used for providing flexible optical apertures. Liquid crystal arrays, however, do not allow full open transmission or full blocking of light, which leads to inefficiency and/or light leakage. Liquid crystal arrays also cause stray light and/or scatter light, cause optical aberrations, and may provide poor transmission and/or limited lifetime at short wavelengths (e.g., UV, DUV, and VUV).
Tilt mirror arrays have been used for providing flexible optical apertures. Tilt mirror arrays, however, do not allow full open transmission, which causes inefficiency. In addition, tilt mirror arrays can cause stray or scattered light, cause optical aberrations, and/or can have limitations on light power density or contamination with DUV light. Tilt mirror arrays also may not allow efficient incorporation of shapes with spectral, polarizing, or phase properties.
Aperture patterns have been exposed and developed on photofilm systems. Photofilm systems, however, do not allow fully open and fully blocked transmission, require time for exposure and development, and have a low damage threshold. Photofilm systems may also degrade and cause contamination when used with UV, DUV, or VUV light systems and may not allow efficient incorporation of shapes with spectral, polarizing, or phase properties.
Aperture patterns have also been applied onto transmissive substrates using inkjet printing. Inkjet printing systems, however, does not allow full open transmission and requires substrate recleaning or a consummable substrate. Inkjet printing systems may also have a low damage threshold and degrade or cause contamination when used with UV, DUV, or VUV light systems. Inkjet printing systems also may not allow efficient incorporation of shapes with spectral, polarizing, or phase properties.
Fourier filters (e.g., plurality of metal bars with adjustable spacing) have also been used for providing flexible optical apertures. U.S. Pat. No. 5,970,168, which is incorporated by reference as if fully set forth herein, describes an example of a Fourier filter. Fourier filters, however, allow for very few shapes and are only mostly useful to block diffraction patterns.
Yet another system for providing flexible optical apertures is the use of a microshutter array. For example, an array of microelectromechanical system shutters may allow for full programmability of pixels (such as those used on the James-Webb space telescope). Microshutter arrays, however, may not allow full open transmission due to the array structure and the array structure may cause stray light. Microshutters are also complex in nature which can add cost and reduce reliability.
As described above, previous flexible optical aperture systems have several disadvantages including, but not limited to, disadvantages such as optical transmission losses, incomplete optical blocking, straylight, optical aberrations, limited shape flexibility or spatial resolution, low damage threshold and DUV incompatibility, limited flexibility to incorporate additional optical features (such as apodization, phase plates, shaped polarizers, and spectral apertures), and incompatibility with existing optical systems. Thus, there is still a need for programmable and adjustable (e.g., flexible) optical aperture systems and methods that provide high optical transmission in combination with complete optical blocking, no optical aberrations, and minimal stray light while allowing incorporation of attenuation apertures, phase apertures, spectral apertures, and polarizing apertures. In addition, the flexible optical aperture systems and methods may have a high damage threshold and compatibility with various optical systems (e.g., UV, DUV, VUV, and EUV optics).